1. Field of the Invention
This invention relates to an improved air-to-air heat pump and, more particularly, to a solar assisted air-to-air heat pump system.
2. Description of the Prior Art
In the conventional heat pump in the heating cycle high-pressure, high-temperature refrigerant vapor is discharged from the compressor and passes to an indoor condenser where the air to be heated extracts heat from the condenser as the air passes over it. As this heat is extracted, the refrigerant is condensed to a liquid and passes through an expansion valve whereby the pressure is reduced and the refrigerant turns into a vapor. The refrigerant is then in a condition to absorb the latent heat of vaporization from an external source in the conventional evaporator. From the evaporator, the refrigerant vapor returns to the compressor to complete the cycle.
An air-to-air heat pump in the heating mode uses the evaporator coil to absorb heat from outside air as the refrigerant vaporizes. The indoor condenser releases heat to the space to be heated.
In the cooling mode, the function of the coils is reversed by means of a reversing valve which reverses the flow of refrigerant. Heat is picked up by the indoor coil (now the evaporator coil) from the air to be cooled and is discharged by the outdoor or condensor coil to the outside air.
An air-to-air heat pump in the heating cycle loses efficiency as the outdoor temperature falls. That is shown by the coefficient of performance which is the relationship of electricity used to heat produced. For example, the coefficient of performance of a typical 3-ton heat pump at 45.degree. F. equals 3; it takes 4100 watts to produce 42,000 BTU's of heat. At 5.degree. F., the coefficient of performance is 1.83 or 3075 watts to produce 19,200 BTU's. The decrease in the coefficient of performance as the outdoor temperature drops is a result of the refrigerant being unable to absorb as much heat from the lower temperature air as the refrigerant changes from liquid to vapor. As the evaporator coil becomes colder, the expansion valve decreases or throttles down the flow of liquid refrigerant to the evaporator coil, thereby reducing the efficiency of the system.
The use of solar collectors in conjunction with combination heat pump and forced-air heating systems are known in the art. The collector heats a liquid which is stored in a thermal collector, such as a hot water storage tank. The heat pump is used as an auxiliary heater. An auxiliary resistance-type heater is sometimes used in such a system. The patent to Ramey, U.S. Pat. No. 4,005,583, issued Feb. 1, 1977, discloses a combination heat pump and low temperature solar heat collector. Such systems are expensive and not very efficient.